1. Field of the Invention
The present invention relates to a photomask used for forming an electrode portion in a high-power semiconductor laser element used in an optical reading/recording drive for CD, CD-R/RW (Recordable/Rewritable), etc., especially in a window-structure semiconductor laser element having a window region. Further, the present invention relates to a method for producing a semiconductor laser element using such a photomask. Furthermore, the present invention relates to a semiconductor laser element produced using such a photomask.
2. Description of the Related Art
FIG. 9 is a perspective view showing a semiconductor laser element 1 having a window region.
The semiconductor laser element 1 includes a small rectangular laser element main body 2 which is obtained by separating a substrate (wafer) including a laser element structure. One of the side surfaces of the laser element main body 2 has a low reflectance so as to act as a laser light emitting face 3 from which laser light L is emitted. Another face of the laser element main body 2 which is opposite to the laser light emitting face 3 has a high reflectance so as to act as a laser light reflecting face 4. Each of the laser light emitting face 3 and the laser light reflecting face 4 is provided with a thin window region 5 for efficiently transmitting the laser light L.
On the upper surface of the laser element main body 2, an electrode portion 6 is formed such that a portion of the electrode portion 6 is superposed on the laser element to include covering an inner portion of the upper surfaces of the window regions 5 (without covering the entire upper surfaces of the window regions 5). formed on the laser light emitting face 3 and the laser light reflecting face 4.
In the semiconductor laser element 1 having such a structure, due to the window regions 5 formed on the laser light emitting face 3 and the laser light reflecting face 4, absorption by the faces 3 and 4 of laser light generated in an active layer inside the chip-shaped laser element main body 2 is reduced, and as a result, damages caused in the faces 3 and 4 can be prevented.
The chip-shaped semiconductor laser element 1 having the window regions 5 is produced by: forming a large number of element regions, which will be semiconductor laser elements 1, on a single substrate in a predetermined pattern; forming electrode portions corresponding to the respective element regions over the substrate; and separating the substrate into pieces of the respective element regions.
FIG. 10 shows a substrate 100 for laser elements (hereinafter, referred to as “laser substrate 100”) on which, after an element structure is formed at a predetermined position, a metal film which will work as an electrode is formed in a predetermined pattern for each element region over the element structure, and a resist is applied over the metal film.
Over the laser substrate 100, a plurality of element regions, each of which will be a semiconductor laser element having window regions, are formed in a predetermined matrix pattern. A window region pattern 101, which will be window regions, is formed by a plurality of stripes extending along the longitudinal direction of the laser substrate 100 with predetermined intervals therebetween along the width direction of the laser substrate 100. After the window region pattern 101 has been formed over the laser substrate 100, a metal film which will function as an electrode, and a resist for patterning the metal film, are formed such that each layer has a predetermined thickness. A portion of the structure on the laser substrate 100, on which the metal film and resist are formed, cannot be observed from outside due to the metal film and resist. However, as shown in FIG. 10, a window region 101 is exposed at one side of the laser substrate 100 so that the position of the window region 101 can be observed through an infrared microscope.
On the upper surface of the resultant structure over the laser substrate 100, a photomask having a predetermined electrode pattern for forming electrode portions corresponding to the respective element regions is provided. This photomask is provided while the window region 101 exposed at the one side of the laser substrate 100 and the photomask are aligned with respect to each other by sight through an infrared microscope.
After the photomask has been aligned so as to be placed at an appropriate position where the window region 101 is provided, an exposure step and development step are performed to pattern the resist such that only a portion of the photomask over which the electrode pattern is formed is left unetched, and then, the photomask is removed. Then, the resist on which the electrode pattern has been formed is used as a mask to perform etching on the metal film, whereby an electrode portion 6 having a desired pattern is formed. Thereafter, the resist is removed.
After the formation of the electrode portion 6, the laser substrate 100 is separated into respective chip-shaped laser elements 1. As shown in FIG. 11, the laser substrate 100 is first separated along the stripe-shaped window region 101 into a plurality of bars, and each bar is further separated along a direction perpendicular to the window region 101, into predetermined, rectangular chip-shaped semiconductor laser elements 1.
In the window region 5 of the semiconductor laser element 1, a gain cannot be obtained with respect to laser light emission. Thus, if the window region 5 is large, an effective resonator length becomes short, and as a result, the I-L characteristic (current-optical output characteristic) of the laser light L emitted from the semiconductor laser element 1 decreases. On the other hand, the window region 5 does not function as a window region unless the window region 5 has a thickness of about 15 μm or more from the faces 3 and 4 of the semiconductor laser element 1. Thus, the window region 5 of the semiconductor laser element 1 typically has a thickness of about 20 to 30 μm or more from the faces 3 and 4 of the semiconductor laser element 1.
On the other hand, each stripe of the window region pattern 101 formed on the laser substrate 100 has a width of about 40 to 60 μm. The electrode pattern (electrode pattern segments) is formed at an interval of 20 to 30 μm.
According to a conventional method, at a step of aligning a photomask for forming electrode portions 6 over the window region pattern 101 on the laser substrate 100, alignment is performed by observation through an infrared microscope, such that the center line of each stripe of the window region pattern 101 with respect to (i.e., “perpendicular to”) its width direction is aligned with the center line of an area between adjoining rows of electrode pattern segments, which is also a separation line along which the laser substrate 100 is separated into bars at a separation step performed later. In this conventional method, an error (deviation) of alignment is not quantitatively evaluated. As a result, it is difficult to align a photomask with respect to the window region pattern 101 of the laser substrate 100 with a sufficient alignment accuracy.